A hot-dip aluminum-based alloy-coated steel sheet (hereinafter referred to as a “hot-dip Al-based alloy-coated steel sheet”) includes a steel sheet whose surface is coated with an alloy, which contains aluminum (Al) as a main component, by a hot-dip method so that the steel sheet can have higher corrosion resistance and/or higher heat resistance. Such a hot-dip Al-based alloy-coated steel sheet has been widely used mainly for members that are required to have heat resistance, such as exhaust gas members of automobiles and members of combustion devices.
Note that the hot-dip Al-based alloy-coated steel sheet has a coated layer having a surface on which a spangle pattern appears, the spangle pattern being formed due to dendrites, which are structures obtained by solidification of Al. The spangle pattern is a characteristic geometric pattern or a flower pattern, and each region (i.e., spangle) of the spangle pattern is constituted by dendrites.
A spangle grows during solidification of Al after coating. Growth of the spangle progresses as below. First, the nucleus of the spangle (i.e., spangle nucleus) occurs. Then, a primary dendrite arm grows from the spangle nucleus. Subsequently, a secondary dendrite arm develops from the primary dendrite arm. Growth of such dendrite arms stops due to a collision between adjacent spangles. It follows that presence of more spangle nuclei in the coated layer causes an increase in number of spangles. This causes each spangle to have a minute size.
The presence of such a spangle does not adversely affect a quality (e.g., corrosion resistance) of the hot-dip Al-based alloy-coated steel sheet. Note, however, that in the market, a hot-dip Al-based alloy-coated steel sheet is preferred which has spangles having a minute size and thus has a surface skin having an inconspicuous spangle pattern.
Under the circumstances, proposed is, for example, a method of producing a hot-dip aluminum-zinc alloy-coated steel sheet which includes a coated layer made of an aluminum-zinc alloy. According to this method, for the purpose of formation of fine spangles, titanium (Ti), zirconium (Zr), niobium (Nb), boron (B), a boride such as aluminum boride (AlB2 or AlB12), titanium carbide (TiC), titanium boride (TiB2), or titanium aluminide (TiAl3) is added to a coating bath so that more substances each acting as a spangle nucleus are obtained. Such a method is disclosed in, for example, Patent Literatures 1 to 3.